Lithium niobate single crystal substrate and method of producing the same

ABSTRACT

To provide a lithium niobate (LN) substrate which allows treatment conditions regarding a temperature, a time, and the like to be easily managed and in which an in-plane distribution of a volume resistance value is very small, and a method of producing the same. 
     A method of producing an LN substrate by using an LN single crystal grown by the Czochralski process, in which an LN single crystal having a Fe concentration of more than 1000 mass ppm and 2000 mass ppm or less in the single crystal and processed into a form of a substrate is buried in an Al powder or a mixed powder of Al and Al 2 O 3 , and heat-treated at a temperature of 550° C. or more and 600° C. or less, to produce a lithium niobate single crystal substrate having a volume resistivity controlled to be within a range of 1×10 8  Ω·cm or more to 1×10 10  Ω·cm or less.

TECHNICAL FIELD

The present invention relates to a lithium niobate single crystalsubstrate used in surface acoustic wave devices and the like, and moreparticularly to improvements in a lithium niobate single crystalsubstrate that is unlikely to cause a decrease in yield in devicefabrication processes, and a method of producing the same.

BACKGROUND ART

Lithium niobate (LiNbO₃: hereinafter, abbreviated as LN) single crystalsare artificial ferroelectric crystals having a melting point ofapproximately 1250° C. and a Curie temperature of approximately 1140° C.In addition, a lithium niobate single crystal substrate (hereinafter,referred to as an LN substrate) made of LN single crystals is utilizedas a material for a surface acoustic wave device (SAW filter) used forremoving noises from electrical signals used mainly in mobilecommunications devices.

The SAW filter has a structure in which a comb electrode is formed of ametal thin film of an AlCu alloy or the like on a substrate formed of apiezoelectric material such as an LN single crystal. This comb electrodeplays an important role that affects the properties of the device. Inaddition, the comb electrode is formed by first forming a metal thinfilm on a piezoelectric material by a sputtering method or the like, andthen etching and removing an unnecessary portion while leaving a combpattern by a photolithographic technique.

In addition, the LN single crystal, serving as the material for the SAWfilter, is industrially obtained by the Czochralski process, in whichthe LN single crystal is generally grown in an electric furnace havingan atmosphere of a nitrogen-oxygen mixed gas in which the concentrationof oxygen is approximately 20% by using a platinum crucible, and thencooled at a predetermined cooling rate in the electric furnace, andthereafter taken out of the electric furnace.

The LN single crystal thus grown is colorless and transparent or takeson a pale yellow color with a high transparency. After the growth, toremove residual strain due to the thermal stress during the growth, theLN single crystal is subjected to heat treatment under soaking at atemperature close to its melting point, and is subjected to polingtreatment for obtaining single polarity, that is, to a series oftreatments in which the temperature of the LN single crystal isincreased from room temperature to a predetermined temperature of theCurie temperature or more, a voltage is then applied to the singlecrystal, and the temperature of the LN single crystal is decreased to apredetermined temperature of the Curie temperature or less while keepingapplying the voltage, thereafter, the application of the voltage isstopped, and the LN single crystal is cooled down to room temperature.After the poling treatment, the LN single crystal (ingot), which hasbeen abraded on its peripheral surface in order to adjust the externalshape of the single crystal, undergoes machining such as slicing,lapping, and polishing steps to become an LN substrate. The substratefinally obtained is substantially colorless and transparent, and has avolume resistivity of approximately 1×10¹⁵ Ω·cm or more.

The LN substrate obtained by such a conventional method has a problem ofpyroelectric breakdown in the process of manufacturing the SAW filter.The pyroelectric breakdown is a phenomenon in which due to thepyroelectric property, which is a property of the LN single crystal,electrical charge is charged up on the surface of the LN substratebecause of the change in temperature applied by the process, andgenerates sparks, which cause the comb electrode formed on the surfaceof the LN substrate to broken, and further cause crack and the like tobe generated in the LN substrate. The pyroelectric breakdown is a majorfactor of causing a decrease in yield in the device fabrication process.In addition, the high light transmittance of the substrate causes also aproblem that light transmitted through the substrate in thephotolithographic process, which is one of the device fabricationprocesses, is reflected at the back surface of the substrate to returnthe front surface, causing the resolution of the formed pattern todeteriorate.

In view of this, to solve this problem, Patent Document 1 proposes amethod in which an LN substrate is exposed to a chemical reducingatmosphere of a gas selected from argon, water, hydrogen, nitrogen,carbon dioxide, carbon monoxide, and oxygen, as well as combinations ofthese at a temperature within a range of 500 to 1140° C. to be blackenedto lower the resistance value of the LN substrate, thereby reducing thepyroelectric property. Note that performing the above-described heattreatment causes the LN crystal, which has been colorless andtransparent, to become colored and opaque. Since the color tone of thecolored and opaque crystal then observed looks brown to black with atransmitted light, the phenomenon in which a crystal becomes colored andopaque is herein referred to as “blackening”. It is considered that theblackening phenomenon occurs because oxygen defects (voids) areintroduced into the LN substrate by the reduction treatment and a colorcenter is thus formed. It is considered that the change in resistancevalue occurs because the valence of Nb ions changes from 5+ to 4+ andfree electrons emitted from Nb ions increase in the substrate tocompensate for deviation of the charge balance due to the generation ofoxygen defects. Accordingly, the degree of blackening and the resistancevalue are substantially proportional to each other.

Meanwhile, since the method described in Patent Document 1 includesheating the LN substrate (crystal) to a high temperature of 500° C. ormore, the treatment time is short; however, variations are likely tooccur in blackening between treatment batches. In addition, colornon-uniformity due to the blackening, that is, in-plane distribution ofresistivity is likely to occur in the heat-treated substrate. Thus,there is a problem that a decrease in yield in the device fabricationprocess still cannot be sufficiently prevented.

For this reason, as a method to solve the above-described problems,Patent Document 2 proposes a method in which an LN substrate (crystal)is heat-treated at a low temperature of 300° C. or more and less than500° C. in a state where the LN substrate (crystal) is buried in apowder formed of at least one element selected from the group consistingof Al, Ti, Si, Ca, Mg, and C.

CONVENTIONAL ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent Application Publication No. 11-92147

Patent Document 2: Japanese Patent Application Publication No.2005-179177

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

In addition, the method described in Patent Document 2 has been a methodthat is surely capable of providing an LN substrate in which colornon-uniformity due to blackening, that is, in-plane distribution ofvolume resistivity is small and the pyroelectric property is suppressed,by using a simple device because the treatment temperature is as low asless than 500° C.

However, since the LN single crystal (having a melting point of 1250°C.) has such a property as to be more easily reduced than a lithiumtantalate single crystal (having a melting point of 1650° C.) which isalso utilized in the SAW filter, a slight change in treatment conditionslargely affects the volume resistivity of the LN crystal (substrate)after the treatment even when the reduction treatment is conducted at alow temperature of less than 500° C. For this reason, in the methoddescribed in Patent Document 2, it is required that treatment conditionsregarding a temperature, a time, and the like be strictly managed.Moreover, even when the treatment is conducted under strictly managedconditions, there is a likelihood that color non-uniformity due toblackening, which is slight though, that is, in-plane distribution ofvolume resistivity occur. Thus, there is still room for furtherimprovements in the method described in Patent Document 2.

The present invention has been made with focus on such problems, and anobject thereof is to provide an LN substrate which make it possible toease the management of treatment conditions regarding a temperature, atime, and the like, and in which color non-uniformity due to blackening,that is, in-plane distribution of volume resistance value is very small,and a method of producing the same.

Means for Solving the Problems

In view of this, the present inventors grew an LN single crystalcontaining a trace of Fe and attempted experiments of reducing the LNsingle crystal. According to this, the present inventors confirmed thatthe LN crystal (substrate) containing Fe had a property that it was lesslikely to be reduced than an LN crystal (substrate) not containing Fe,and further found that the higher the content of Fe contained in thecrystal is, the less likely the LN crystal (substrate) is to be reducedsubstantially in proportion. The present invention has been completedbased on such findings.

Specifically, a first aspect of the present invention is a lithiumniobate single crystal substrate, wherein

a volume resistivity of the lithium niobate single crystal substrate iscontrolled to be within a range of 1×10⁸ Ω·cm or more to 1×10¹⁰ Ω·cm orless, and

a Fe concentration in a lithium niobate single crystal is more than 1000mass ppm and 2000 mass ppm or less.

In addition, a second aspect of the present invention is a method ofproducing a lithium niobate single crystal substrate by using a lithiumniobate single crystal grown by the Czochralski process, wherein

a lithium niobate single crystal having a Fe concentration of more than1000 mass ppm and 2000 mass ppm or less in the single crystal andprocessed into a form of a substrate is buried in an Al powder or amixed powder of Al and Al₂O₃, and heat-treated at a temperature of 550°C. or more and 600° C. or less, to produce a lithium niobate singlecrystal substrate having a volume resistivity controlled to be within arange of 1×10⁸ Ω·cm or more to 1×10¹⁰ Ω·cm or less.

Next, a third aspect of the present invention is the method of producinga lithium niobate single crystal substrate described in the secondaspect, wherein

an arithmetic average roughness Ra of a surface of the lithium niobatesingle crystal processed into the form of the substrate is 0.2 μm ormore and 0.4 μm or less.

A fourth aspect of the present invention is the method of producing alithium niobate single crystal substrate described in the second aspector the third aspect, wherein

the heat treatment is conducted in a vacuum atmosphere or in areduced-pressure atmosphere of an inert gas.

Moreover, a fifth aspect of the present invention is the method ofproducing a lithium niobate single crystal substrate described in thesecond aspect, the third aspect, or the fourth aspect, wherein

the heat treatment is conducted for 1 hour or more.

Effects of the Invention

According to the lithium niobate single crystal substrate of the firstaspect, since the Fe concentration in the lithium niobate single crystalis more than 1000 mass ppm and 2000 mass ppm or less, the volumeresistivity after the reduction treatment is stably controlled to bewithin the range of 1×10⁸ Ω·cm or more to 1×10¹⁰ Ω·cm or less.

In addition, according to the method of producing a lithium niobatesingle crystal substrate of the second aspect to the fifth aspect, sincea lithium niobate single crystal having a Fe concentration of more than1000 mass ppm and 2000 mass ppm or less in the single crystal andprocessed into the form of a substrate is buried in an Al powder or amixed powder of Al and Al₂O₃, and heat-treated at a temperature of 550°C. or more and 600° C. or less, it is possible to stably produce alithium niobate single crystal substrate in which a volume resistivityafter the reduction treatment is controlled to be within a range of1×10⁸ Ω·cm or more to 1×10¹⁰ Ω·cm or less and an in-plane distributionof the volume resistivity is also small.

Modes for Practicing the Invention

The present invention is described below in detail.

(1) Volume Resistivity and Pyroelectric Effect (Pyroelectric Property)of LN Single Crystal

The LN single crystal changes in volume resistivity and color (lighttransmittance spectrum) in accordance with the concentration of oxygendefects present in the crystal. Specifically, once oxygen defects areintroduced into the LN single crystal, the valence of part of Nb ionschanges from 5+ to 4+ due to the necessity of compensating for thecharge balance due to the defects of oxygen ions with a valence of −2,causing the volume resistivity to change. In addition, a color centerattributable to the oxygen defects is generated to cause absorption oflight.

The change in volume resistivity is considered to occur becauseelectrons, which are carriers, migrate between Nb⁵⁺ ions and Nb⁴⁺ ions.The volume resistivity of the crystal is determined by the product ofthe number of carriers per unit volume and the mobility of the carriers.If the mobility is the same, the volume resistivity is proportional tothe number of oxygen voids. Color change due to the absorption of lightis considered to be caused by a color center which is formed due toelectrons in a metastable state captured in oxygen defects.

The above-described control on the number of oxygen voids may beconducted by so-called heat treatment in atmosphere. The concentrationof oxygen voids in the crystal placed at a specific temperature changesin a manner of coming into equilibrium with the oxygen potential (oxygenconcentration) of the atmosphere in which the crystal is placed. If theoxygen concentration of the atmosphere becomes lower than theequilibrium concentration, the concentration of oxygen voids in thecrystal increases. In addition, increasing the temperature with aconstant oxygen concentration of the atmosphere allows the concentrationof oxygen voids to increase even when the oxygen concentration of theatmosphere is made lower than the equilibrium concentration.Accordingly, to make the concentration of oxygen voids higher to enhancethe opacity, the temperature may be set higher and the oxygenconcentration of the atmosphere may be made lower.

Since the LN single crystal has a strong ion-binding property, the voidsdisperse at a relatively high speed. However, the change in theconcentration of oxygen voids requires the in-crystal dispersion ofoxygen, and hence, the crystal needs to be retained in the atmospherefor a certain period of time. This speed of dispersion depends greatlyon the temperature, and the concentration of oxygen voids does notchange in an actual period of time at near room temperature.Accordingly, to obtain an opaque LN crystal in a short period of time,the crystal needs to be retained in an atmosphere having a low oxygenconcentration at a temperature that allows a sufficient oxygendispersion speed to be achieved. Cooling down the crystal promptly afterthe treatment allows the crystal in which the concentration of oxygenvoids introduced at a high temperature is maintained to be obtained atroom temperature.

Here, the pyroelectric effect (pyroelectric property) is attributable tothe deformation of lattice caused by change in temperature of thecrystal. It is understood that in a crystal having electric dipoles, thepyroelectric effect occurs because the distance between the dipoleschanges depending on the temperature. The pyroelectric effect occursonly in materials having high electrical resistances. The displacementof ions causes an electrical charge in the dipole direction on thesurface of the crystal. However, in a material having a low electricalresistance, this electrical charge is neutralized because of theelectrical conductivity possessed by the crystal itself. Since a normaltransparent LN single crystal has a volume resistivity at the level of1×10¹⁵ Ω·cm as described above, the pyroelectric effect is markedlyexhibited. However, a blackened opaque LN single crystal has a volumeresistivity significantly decreased by several orders of magnitude, andaccordingly, the electrical charge is neutralized within a very shortperiod of time, so that the pyroelectric property is no longerexhibited.

(2) Method of Producing LN Substrate According to the Present Invention

The method described in Patent Document 1, which aims to decrease thepyroelectric property of an LN substrate, has problems that since the LNsubstrate is heated to a high temperature of 500° C. or more asdescribed above, although the treatment time is short, variations arelikely to occur in blackening between treatment batches, and colornon-uniformity due to the blackening, that is, in-plane distribution ofresistivity occurs in the heat-treated substrate.

In addition, the method described in Patent Document 2, which aims tosolve the problems of Patent Document 1, is a method that makes itpossible to provide an LN substrate in which color non-uniformity due toblackening, that is, in-plane distribution of volume resistivity issmall and the pyroelectric property is suppressed, because the treatmenttemperature is as low as less than 500° C.

Nonetheless, since the LN single crystal (having a melting point of1250° C.) has such a property as to be more easily reduced than alithium tantalate single crystal (LT single crystal), which has amelting point of 1650° C., and a slight change in treatment conditionsaffects the volume resistivity of the LN substrate after the treatment,the method described in Patent Document 2 requires that the treatmentconditions be strictly managed and there is still room for furtherimprovements.

In the meantime, the results of experiments conducted by the presentinventors revealed that the reducibility (reduction easiness) of the LNsingle crystal is decreased (made difficult to be reduced) by adding atrace of Fe into the crystal, and the reducibility of the LN singlecrystal is further decreased as the content of Fe increases.

Specifically, the method of producing an LN substrate according to thepresent invention has been made with attention to the above-describedreducibility of the LN single crystal, and is characterized in that themethod produces an LN substrate having a volume resistivity controlledto be within a range of 1×10⁸ Ω·cm or more to 1×10¹⁰ Ω·cm or less byheat-treating an LN single crystal (LN substrate) having a Feconcentration of more than 1000 mass ppm and 2000 mass ppm or less inthe single crystal and processed into a form a substrate in a reducingatmosphere.

Note that the LN substrate to be heat-treated is preferably a substrateobtained by processing an LN single crystal after a poling treatment.

In addition, the heat treatment on the LN substrate to which a trace ofFe has been added is conducted by using a metal Al powder having a lowfree energy of oxide formation in order to promote the reductionreaction at a low temperature. Specifically, the LN substrate to which atrace of Fe has been added is buried in a metal Al powder or a mixedpowder of a metal Al powder and an Al₂O₃ powder, and is heat-treated.Here, the heating temperature for the LN substrate is 550° C. or moreand 600° C. or less. In addition, the atmosphere for the heat treatmentis preferably vacuum or an inert gas (such as a nitrogen gas or an argongas), and the treatment time is preferably 1 hour or more. Further, thearithmetic average roughness Ra of the surface of the LN substrate ispreferably set at 0.2 μm or more and 0.4 μm or less in order to increasethe surface area of the LN substrate to promote its reduction reaction.

Moreover, as the most preferable conditions with the controllability ofthe treatment step, the properties of the finally obtained substrate,the uniformity of these properties, the reproducibility, and the liketaken into consideration, it is effective to use an LN substrate cut outof an LN crystal ingot after poling treatment, to bury the LN substratein a mixed powder of Al and Al₂O₃, and to perform the heat treatment inan atmosphere of an inert gas such as a nitrogen gas or an argon gas orin an atmosphere like vacuum. Note that, the vacuum atmosphere is moredesirable than the inert gas atmosphere because the vacuum atmosphereallows the blackening treatment of the LN substrate in a relativelyshorter period of time.

(3) Method of Judging the Effect of the “Heat Treatment” According tothe Present Invention

As a practical method of judging the effect of the “heat treatment”,that is, whether no pyroelectric property is observed in an LNsubstrate, a heat cycle test in which temperature changes which the LNsubstrate undergoes are simulated to occur is useful. Specifically, theLN substrate is placed on a hot plate heated to 80° C., and heat cycleis applied to the LN substrate. As a result, in the case of the LNsubstrate which has not been subjected to the blackening treatment, ahigh potential of 10 kV or more is generated on the surface of the LNsubstrate, and sparks are observed. On the other hand, in the case ofthe LN substrate which has been blackened by the “heat treatment”according to the present invention, the surface potential of the LNsubstrate is only at the level of up to several hundred V, and thephenomenon of sparking on the surface of the LN substrate is notobserved at all. Therefore, the judgment on whether the LN substrate hasbeen blackened or not is useful as a practical method of judging thepyroelectric property.

EXAMPLES

Next, Examples of the present invention are specifically described alsoby giving Comparative Examples.

Example 1

A Fe-doped LN single crystal having a diameter of 4 inches was grown bythe Czochralski process using a raw material having a congruentcomposition. The growth atmosphere was a nitrogen-oxygen mixed gashaving an oxygen concentration of approximately 20%. The concentrationof Fe doped in the crystal was set at 1100 ppm. The crystal thusobtained was red in color.

This crystal was subjected to the heat treatment for removing theresidual thermal strain under soaking and the poling treatment formaking it single-polarized. Thereafter, the crystal was abraded on itsperipheral surface in order to adjust the external shape of the crystal,and then sliced into an LN substrate.

The LN substrate thus obtained was buried in an Al powder, and was thenheat-treated at 600° C. for 20 hours in a vacuum atmosphere.

The LN substrate after the heat treatment was dark green brown in color,had a volume resistivity of approximately 1×10⁸ Ω·cm, and the variation(σ/Ave) in volume resistivity in the plane of the substrate was lessthan 3%. It was also visually observed that color non-uniformity did notoccur. Here, Ave is an average value of volume resistivities measured atone point in the center portion and four points on the outer peripheralportion, five points in the surface in total, of the substrate, and σ isa standard deviation of these. Note that the volume resistivity wasmeasured by the three-terminal method according to JIS K-6911.

Next, a heat cycle test was conducted in which the LN substrate kept atroom temperature was placed on an 80° C. hot plate. As a result, thesurface potential generated immediately after the substrate was placedon the hot plate was 10 V or less, and the phenomenon of sparking on thesurface of the LN substrate was not observed.

In addition, the LN substrate thus obtained had a Curie temperature of1140° C., and values of the physical properties that affect theproperties of SAW filters were not different from those of conventionalproducts that have not been subjected to the blackening treatment.

Example 2

The heat treatment was conducted under substantially the same conditionsas those in Example 1 except that the heat treatment temperature waschanged to 550° C.

The LN substrate thus obtained was dark green brown in color, had avolume resistivity of approximately 3×10⁸ Ω·cm, and the variation(σ/Ave) in volume resistivity in the plane of the substrate was lessthan 3%. It was also visually observed that color non-uniformity did notoccur.

In addition, in the heat cycle test, the surface potential generatedimmediately after the LN substrate was placed on the hot plate was 30 Vor less, and the phenomenon of sparking on the surface of the LNsubstrate was not observed.

Example 3

The heat treatment was conducted under substantially the same conditionsas those in Example 1 except that the atmosphere was changed to anitrogen gas atmosphere.

The LN substrate thus obtained was dark green brown in color, had avolume resistivity of approximately 2×10⁸ Ω·cm, and the variation(σ/Ave) in volume resistivity in the plane of the substrate was lessthan 3%. It was also visually observed that color non-uniformity did notoccur.

In addition, in the heat cycle test, the surface potential generatedimmediately after the LN substrate was placed on the hot plate was 20 Vor less, and the phenomenon of sparking on the surface of the LNsubstrate was not observed.

Example 4

The heat treatment was conducted under substantially the same conditionsas those in Example 1 except that the heat treatment time for the LNsubstrate was changed to 1 hour.

The LN substrate thus obtained was brown in color, had a volumeresistivity of approximately 1×10¹⁰ Ω·cm, and the variation (σ/Ave) involume resistivity in the plane of the substrate was less than 3%. Itwas also visually observed that color non-uniformity did not occur.

In addition, in the heat cycle test, the surface potential generatedimmediately after the LN substrate was placed on the hot plate was 100 Vor less, and the phenomenon of sparking on the surface of the LNsubstrate was not observed.

Example 5

The heat treatment was conducted under substantially the same conditionsas those in Example 1 except that the concentration of Fe doped in theLN crystal was changed to 2000 ppm.

The LN substrate thus obtained was dark green brown in color, had avolume resistivity of approximately 5×10⁸ Ω·cm, and the variation(σ/Ave) in volume resistivity in the plane of the substrate was lessthan 3%. It was also visually observed that color non-uniformity did notoccur.

In addition, in the heat cycle test, the surface potential generatedimmediately after the LN substrate was placed on the hot plate was 40 Vor less, and the phenomenon of sparking on the surface of the LNsubstrate was not observed.

Example 6

The heat treatment was conducted under substantially the same conditionsas those in Example 2 except that the concentration of Fe doped in theLN crystal was changed to 2000 ppm.

The LN substrate thus obtained was dark green brown in color, had avolume resistivity of approximately 5×10⁹ Ω·cm, and the variation(σ/Ave) in volume resistivity in the plane of the substrate was lessthan 3%. It was also visually observed that color non-uniformity did notoccur.

In addition, in the heat cycle test, the surface potential generatedimmediately after the LN substrate was placed on the hot plate was 90 Vor less, and the phenomenon of sparking on the surface of the LNsubstrate was not observed.

Example 7

The heat treatment was conducted under substantially the same conditionsas those in Example 3 except that the concentration of Fe doped in theLN crystal was changed to 2000 ppm.

The LN substrate thus obtained was dark green brown in color, had avolume resistivity of approximately 7.7×10⁸ Ω·cm, and the variation(σ/Ave) in volume resistivity in the plane of the substrate was lessthan 3%. It was also visually observed that color non-uniformity did notoccur.

In addition, in the heat cycle test, the surface potential generatedimmediately after the LN substrate was placed on the hot plate was 50 Vor less, and the phenomenon of sparking on the surface of the LNsubstrate was not observed.

Example 8

The heat treatment was conducted under substantially the same conditionsas those in Example 7 except that the treatment temperature for the LNsubstrate was changed to 550° C.

The LN substrate thus obtained was dark green brown in color, had avolume resistivity of approximately 7.7×10⁹ Ω·cm, and the variation(σ/Ave) in volume resistivity in the plane of the substrate was lessthan 3%. It was also visually observed that color non-uniformity did notoccur.

In addition, in the heat cycle test, the surface potential generatedimmediately after the LN substrate was placed on the hot plate was 100 Vor less, and the phenomenon of sparking on the surface of the LNsubstrate was not observed.

Example 9

The heat treatment was conducted under substantially the same conditionsas those in Example 1 except that the LN substrate was buried in a mixedpowder of 10% by mass of Al and 90% by mass of Al₂O₃.

The LN substrate thus obtained was dark green brown in color, had avolume resistivity of approximately 5×10⁸ Ω·cm, and the variation(σ/Ave) in volume resistivity in the plane of the substrate was lessthan 3%. It was also visually observed that color non-uniformity did notoccur.

In addition, in the heat cycle test, the surface potential generatedimmediately after the LN substrate was placed on the hot plate was 40 Vor less, and the phenomenon of sparking on the surface of the LNsubstrate was not observed.

Example 10

The heat treatment was conducted under substantially the same conditionsas those in Example 2 except that the LN substrate was buried in a mixedpowder of 10% by mass of Al and 90% by mass of Al₂O₃.

The LN substrate thus obtained was dark green brown in color, had avolume resistivity of approximately 2×10⁹ Ω·cm, and the variation(σ/Ave) in volume resistivity in the plane of the substrate was lessthan 3%. It was also visually observed that color non-uniformity did notoccur.

In addition, in the heat cycle test, the surface potential generatedimmediately after the LN substrate was placed on the hot plate was 80 Vor less, and the phenomenon of sparking on the surface of the LNsubstrate was not observed.

Example 11

The heat treatment was conducted under substantially the same conditionsas those in Example 5 except that the LN substrate was buried in a mixedpowder of 10% by mass of Al and 90% by mass of Al₂O₃.

The LN substrate thus obtained was dark green brown in color, had avolume resistivity of approximately 7.7×10⁹ Ω·cm, and the variation(σ/Ave) in volume resistivity in the plane of the substrate was lessthan 3%. It was also visually observed that color non-uniformity did notoccur.

In addition, in the heat cycle test, the surface potential generatedimmediately after the LN substrate was placed on the hot plate was 100 Vor less, and the phenomenon of sparking on the surface of the LNsubstrate was not observed.

Example 12

The heat treatment was conducted under substantially the same conditionsas those in Example 3 except that the LN substrate was buried in a mixedpowder of 10% by mass of Al and 90% by mass of Al₂O₃.

The LN substrate thus obtained was dark green brown in color, had avolume resistivity of approximately 1×10⁹ Ω·cm, and the variation(σ/Ave) in volume resistivity in the plane of the substrate was lessthan 3%. It was also visually observed that color non-uniformity did notoccur.

In addition, in the heat cycle test, the surface potential generatedimmediately after the LN substrate was placed on the hot plate was 60 Vor less, and the phenomenon of sparking on the surface of the LNsubstrate was not observed.

Example 13

The heat treatment was conducted under substantially the same conditionsas those in Example 7 except that the LN substrate was buried in a mixedpowder of 10% by mass of Al and 90% by mass of Al₂O₃.

The LN substrate thus obtained was dark green brown in color, had avolume resistivity of approximately 1×10¹⁰ Ω·cm, and the variation(σ/Ave) in volume resistivity in the plane of the substrate was lessthan 3%. It was also visually observed that color non-uniformity did notoccur.

In addition, in the heat cycle test, the surface potential generatedimmediately after the LN substrate was placed on the hot plate was 100 Vor less, and the phenomenon of sparking on the surface of the LNsubstrate was not observed.

Comparative Example 1

A LN single crystal having a diameter of 4 inches was grown by theCzochralski process using a raw material having a congruent composition.The growth atmosphere was a nitrogen-oxygen mixed gas having an oxygenconcentration of approximately 20%. The crystal thus obtained was paleyellow in color.

This crystal was subjected to the heat treatment for removing theresidual strain under soaking and the poling treatment for making itsingle-polarized. Thereafter, the crystal was abraded on its peripheralsurface in order to adjust the external shape of the crystal, and thensliced into a substrate.

The LN substrate thus obtained was heat-treated at 800° C. for 1 minutein nitrogen.

The LN substrate after the heat treatment was black in color, but it wasvisually observed that color non-uniformity occurred.

As inferred from the fact that color non-uniformity occurred, althoughthe average of measured values of volume resistivity in the plane of theLN substrate was approximately 1×10⁹ Ω·cm, there were variations (σ/Ave)of approximately 30% at some measured spots.

In addition, in the heat cycle test, the surface potential generatedimmediately after the LN substrate was placed on the hot plate was 60 Vor less, and the phenomenon of sparking on the surface of the LNsubstrate was not observed.

Comparative Example 2

A transparent pale yellow LN single crystal having a diameter of 4inches was grown in the same manner as in Comparative Example 1, and anLN substrate was produced in the same manner as in Comparative Example1.

The LN substrate thus obtained was buried in an aluminum (Al) powder,and was then heat-treated at 480° C. for 20 hours in a nitrogen gasatmosphere.

The LN substrate after the heat treatment was black in color, had avolume resistivity of approximately 1×10⁸ Ω·cm, and the variation(σ/Ave) in volume resistivity in the plane of the substrate was lessthan 3% as in the case of Examples. It was also visually observed thatcolor non-uniformity did not occur.

Next, a heat cycle test was conducted in which the substrate kept atroom temperature was placed on an 80° C. hot plate. As a result, thesurface potential generated immediately after the substrate was placedon the hot plate was 10 V or less, and the phenomenon of sparking on thesurface of the substrate was not observed.

In the meantime, when an LN substrate is heat-treated, conditionsregarding the temperature (480° C.) and the time (20 hours) are strictlymanaged. When this management was neglected, there was a case whereslight color non-uniformity was observed in the LN substrate after thetreatment and the variation (σ/Ave) in volume resistivity in the surfacewas approximately 10%.

TABLE 1-1 Fe Reducing Temperature (ppm) Agent Atmosphere (° C.) Example1 1100 Al Vacuum 600 Example 2 1100 Al Vacuum 550 Example 3 1100 AlNitrogen 600 Example 4 1100 Al Vacuum 600 Example 5 2000 Al Vacuum 600Example 6 2000 Al Vacuum 550 Example 7 2000 Al Nitrogen 600 Example 82000 Al Nitrogen 550 Example 9 1100 Al + Al₂O₃ Vacuum 600 Example 101100 Al + Al₂O₃ Vacuum 550 Example 11 2000 Al + Al₂O₃ Vacuum 600 Example12 1100 Al + Al₂O₃ Nitrogen 600 Example 13 2000 Al + Al₂O₃ Nitrogen 600Comparative — — Nitrogen 800 Example 1 Comparative — Al Nitrogen 480Example 2

TABLE 1-2 Volume Surface Time Resistivity Potential (Hour) (Ω · cm) (V)Example 1 20 1.0 × 10⁸ 10 Example 2 20 3.0 × 10⁸ 30 Example 3 20 2.0 ×10⁸ 20 Example 4  1  1.0 × 10¹⁰ 100 Example 5 20 5.0 × 10⁸ 40 Example 620 5.0 × 10⁹ 90 Example 7 20 7.7 × 10⁸ 50 Example 8 20 7.7 × 10⁹ 100Example 9 20 5.0 × 10⁸ 40 Example 10 20 2.0 × 10⁹ 80 Example 11 20 7.7 ×10⁹ 100 Example 12 20 1.0 × 10⁹ 60 Example 13 20  1.0 × 10¹⁰ 100Comparative 1 min. 1.0 × 10⁹ 60 Example 1 Comparative 20 1.0 × 10⁸ 10Example 2

Evaluation

(1) As acknowledged from the comparison between the volume resistivity(3.0×10⁸ Ω·cm) of the LN substrate according to Example 2 which washeat-treated under substantially the same conditions as those in Example1 (600° C.) except that the heat treatment temperature was changed to550° C. and the volume resistivity (1.0×10⁸ Ω·cm) of the LN substrateaccording to Example 1 described above, and the comparison between thevolume resistivity (2.0×10⁸ Ω·cm) of the LN substrate according toExample 3 which was heat-treated under substantially the same conditionsas those in Example 1 (vacuum atmosphere) except that the atmosphere waschanged to a nitrogen gas atmosphere and the volume resistivity (1.0×10⁸Ω·cm) of the LN substrate according to Example 1 described above, andthe like, it can be understood that difference in treatment conditions(for example, the temperature, the atmosphere, and the like) does notgreatly affect the volume resistivity of an LN substrate containing Fe.

Actually, as compared with the treatment method described in PatentDocument 2 (Comparative Example 2), no great change was observed in thevariations (σ/Ave) in volume resistivity in the surfaces of the LNsubstrates after the treatment even though the heat treatment conditionswere not so strictly managed in Examples 1 to 13.

(2) On the other hand, in Comparative Example 2, it was acknowledgedthat when the management of the above-described heat treatmentconditions was neglected, there was a case where the variation (σ/Ave)in volume resistivity in the plane of the LN substrate after thetreatment was approximately 10% as described above.(3) That is, it is acknowledged that Examples 1 to 13 according to thepresent invention are superior to the treatment method described inPatent Document 2 (Comparative Example 2).

POSSIBILITY OF INDUSTRIAL APPLICATION

The present invention makes it possible to stably obtain an LN substratein which a volume resistivity after reduction treatment is controlled tobe within a range of 1×10⁸ Ω·cm or more to 1×10¹⁰ Ω·cm or less, and anin-plane distribution of the volume resistivity is small. Therefore, thepresent invention has a possibility of industrial application for use asa material for a surface acoustic wave device (SAW filter).

The invention claimed is:
 1. A method of producing a lithium niobatesingle crystal substrate by using a lithium niobate single crystal grownby the Czochralski process, wherein a lithium niobate single crystalhaving a Fe concentration of more than 1000 mass ppm and 2000 mass ppmor less in the single crystal and processed into a form of a substrateis buried in an Al powder or a mixed powder of Al and Al₂O₃, andheat-treated at a temperature of 550° C. or more and 600° C. or less, toproduce a lithium niobate single crystal substrate having a volumeresistivity controlled to be within a range of 1×10⁸ Ω·cm or more to1×10¹⁰ Ω·cm or less, and the variation (σ/Ave) in the volume resistivityin the plane of the lithium niobate single crystal substrate is lessthan 3%, wherein Ave is an average value of volume resistivitiesmeasured at one point in a center portion and four points on an outerperipheral portion, five points in a surface in total, of the lithiumniobate single crystal substrate, and σ is a standard deviation of thevolume resistivities, and the volume resistivities are values measuredby a three-terminal method according to JIS K-6911.
 2. The method ofproducing a lithium niobate single crystal substrate according to claim1, wherein an arithmetic average roughness Ra of a surface of thelithium niobate single crystal processed into the form of the substrateis 0.2 μm or more and 0.4 μm or less.
 3. The method of producing alithium niobate single crystal substrate according to claim 2, whereinthe heat treatment is conducted in a vacuum atmosphere or in areduced-pressure atmosphere of an inert gas.
 4. The method of producinga lithium niobate single crystal substrate according to claim 3, whereinthe heat treatment is conducted for 1 hour or more.
 5. The method ofproducing a lithium niobate single crystal substrate according to claim2, wherein the heat treatment is conducted for 1 hour or more.
 6. Themethod of producing a lithium niobate single crystal substrate accordingto claim 1, wherein the heat treatment is conducted in a vacuumatmosphere or in a reduced-pressure atmosphere of an inert gas.
 7. Themethod of producing a lithium niobate single crystal substrate accordingto claim 6, wherein the heat treatment is conducted for 1 hour or more.8. The method of producing a lithium niobate single crystal substrateaccording to claim 1, wherein the heat treatment is conducted for 1 houror more.